Multi-function receivers for communication and navigation, as well as single-function receivers for communications, surveillance, or reconnaissance, are at times exposed to incident signals of interest having substantially different power levels. Allowing higher level signals into the receiver front-end unattenuated can compromise receiver performance and inhibit or interfere with the reception of lower level signals. Particularly strong signals could even drive the amplifier in a receiver front end into compression or saturation as discussed above—distorting, compressing, and masking weaker signals and thereby desensing the receiver.
The conventional solutions to this dilemma are to insert a fixed or variable resistive attenuator, or a diode limiter, prior to the first amplifier in the receiver front-end in order to limit the maximum power level that the amplifier can be exposed to. While such solutions can prevent larger signals from compressing or saturating the amplifier, they indiscriminately attenuate signal power across a broad band of frequencies—unavoidably attenuating weaker signals as well as stronger signals, raising the receiver noise floor, and introducing additional sources of signal distortion that significantly degrade the dynamic range of the receiver.
An alternative, better, solution would be to introduce a frequency selective bandstop filter, with a fixed level of stopband attenuation, to attenuate stronger signals within its stopband and leave weaker signals outside of its stopband unaffected. Further, such bandstop filters should be frequency agile so that they can be tuned to different frequencies to adapt to changes in the operating frequency of the stronger signals. Conventional bandstop filters suffer significant performance degradation when tuned over a substantial frequency range, making conventional bandstop filter approaches undesirable for realizing frequency-agile frequency-selective attenuators. Recently, compact narrowband absorptive bandstop, or “notch”, filters have been demonstrated that can be tuned over a substantial frequency range without significant performance degradation. Descriptions of such absorptive filters may be found in the following papers, each of which is incorporated herein by reference: D. R. Jachowski, “Passive enhancement of resonator Q in microwave notch filters,” IEEE MTT-S Int. Microw. Symp. Dig., pp. 1315-1318, June 2004 (“Jachowski-1”); D. R. Jachowski, “Compact, frequency-agile, absorptive bandstop filters,” IEEE MTT-S Int. Microw. Symp. Dig., June 2005 (“Jachowski-2”); A. C. Guyette, I. C. Hunter, R. D. Pollard, and D. R. Jachowski, “Perfectly-matched bandstop filters using lossy resonators,” IEEE MTT-S Int. Microw. Symp. Dig., June 2005; D. R. Jachowski, “Cascadable lossy passive biquad bandstop filter,” IEEE MTT-S Int. Microw. Symp. Dig., pp. 1213-1316, June 2006; D. R. Jachowski, “Synthesis of lossy reflection-mode bandstop filters,” in Proc. Int. Workshop on Microwave Filters, CNES, Toulouse, France, 16-18 Oct. 2006; and P. W. Wong, I. C. Hunter, and R. D. Pollard, “Matched Bandstop Resonator with Tunable K-Inverter,” Proc. 37th Eur. Microw. Conf., pp. 664-667, October 2007. While, due to their relative simplicity, “first-order” absorptive filters tend to be the most practical to use in frequency-agile applications, the attenuation characteristics of such first-order sections alone tend to lack sufficient stopband bandwidth to be of practical use. Consequently, first-order sections are cascaded to realize practical stopband bandwidths, e.g. as described in Jachowski-2 and in I. Hunter, A. Guyette, R. D. Pollard, “Passive microwave receive filter networks using low-Q resonators,” IEEE Microw. Mag., pp. 46-53, September 2005, incorporated herein by reference. An absorptive notch filter approach may then be suitable for realizing frequency-agile frequency-selective attenuators.
An even better solution than a frequency-agile frequency-selective attenuator would be one with variable attenuation, so that the attenuation of stronger signals can be tailored to optimize receiver dynamic range. A conventional bandstop filter approach to realizing this variable attenuation function is undesirable because the bandwidth of a conventional bandstop filter is dependent on the level of its stopband attenuation, so that varying one varies the other. There has also been no known means of adjusting the stopband attenuation level of frequency-agile absorptive bandstop filters without undesirably altering their stopband bandwidth or other performance parameters, as for example in Sachihiro Toyoda, “Notch filters with variable center frequency and attenuation,” IEEE MTT-S Int. Microw. Symp. Dig., June 1989. Consequently, the attenuation of stronger signals cannot currently be tailored to their specific power levels and receiver dynamic range is still compromised.
It would therefore be desirable to provide a new method of tuning a frequency-agile absorptive bandstop filter as a means of realizing a frequency-agile frequency-selective variable attenuator, such that stopband attenuation level can be varied while preserving stopband bandwidth, low passband insertion loss, and substantial frequency selectivity.